Slotted cylinder antenna



Aug. 26, 1958 Y e. B. M KlMMlE 2,349,711

SLOTTED CYLINDER ANTENNA Fil ed May 15, 1953 2 Sheets-Sheet 1 INVENTOR.

EEEIREE B. MHEKEMMIE ym I'M d TTORNE I 2,849,711 Patented Aug. 26, 1958SLOTTED CYLINDER ANTENNA I George B. MacKimmie,Montreal, Quebec, Canada,as-

- Sig Ql to Radio Corporation of America, a corporation of DelawareThis'invention relates to antennas and particularly to antennasespecially suitable for broadcasting very-highfrequency'andultra-high-frequency radio signals, such as television picture and soundinformation.

An object of the invention is to provide a simple radiating system forelectromagnetic wave energy whichhas a broadfreq'uency bandcharacteristic suitable for very-highfrequency and ultra-hig'h-frequencyservices.

Another object of the invention is to eliminate complicated transmissionline distribution harnesses where a number of radiating elements areused.

A further object of the invention is to provide an antenna system which,by its size and configuration, eliminates theneed for a separatesupporting tower, and which Within itself provides a means of conveyingthe energy from the transmitter to the antenna with substantially noloss of'power in the transmission line. g

These and other objects are achieved, in accordance with this invention,by an antenna system which includes an elongated conductive cylinder.The cylinder acts as the antenna tower, and as a waveguide transmissionline, and has slots therein which are the radiating elements. Insidethecylinder, a longitudinal fin extends approximately one-half the distanceacross the inside of the tower. When the antenna is cylindrical incross-section, this longitudinal fin is on a radius of the cylinder. Theposition on the circumference of the cylinder chosen for the slots whichact as the radiating elementsis such that substantially no energy iscoupled from the waveguide across the slots (to be interchanged withfree space by the slots) unless deliberate distortion of the internalfield distribution within the cylinder is arranged. The, slotsthemselves arearranged end to end along an element of the cylinderparallel to the axis of the cylinder. For in-phase radiation from allslots, the slots are spaced center to center from each Iother a distancewhich is'a multiple of onehalf wavelength. in the'waveguide at theoperating frequency. v 7

Each slot'is. excited'by a controlled distortion of the internal'fielddistribution within the cylinder near the center of the'flslot; In the.plane in which the slot is to be excited,' conductive elements areconnected to the longi tudinal fin to shiftthe distribution of theelectric field within the guide. One of theseconductive elements extendsfro'rn the'center axis of the cylinder where it attaches to the freeedge of the fin tov a point fonthe circumference ofthe cylinder. Anotherconductive element extendsin aplane at right angles. to the fin-towardthe circumference of the. cylinder but is spaced therefrom.

The finned waveguide-which transfers the energy up the antenna tower ,tothe slotsmay be excited by a probe fed by a coaxialtransmission line.Alternatively, a section of waveguidesuch as a rectangular waveguide maybe used "to couple the energy from the radio frequency apparatus to thefinriedwaveguide' part of the antenna system. I f

A more '1. detailed description follows in 1 conjunctionwiththeaccompanying'drawingwhercini Figure 1 is an elevation ofanantenna array in accordance with the invention;

Figure 2 is across-sectional view along the line 22 of Figure 1 showingthe representative electric field dis-' tribution;

Figure 3 is across-sectional view along the line 3-3 of Figure 1 showingthe representative electric field distribution;

Figure 4 is a cross-sectional view along the line 44 of Figure 1 showingthe representative electric field dis tribution;

Figure 5 is a cross-sectional View. along the line 55 of Figure 1;

Figures 6A and 6B are a top sectional view and a view in perspectiverespectively of one arrangement for exciting energy in the tubularcylinder of Figure 1;

Figure 7' is a cross-sectional view of another method of excitingradiofrequency energy in the tubular cylinder ofFigure 1; and

Figures 8, 9, 10 and 11 show other cross-sectional configurations ofwaveguide with which the principles of the invention may be practiced.-

Referring to Figure 1, there is shown an antenna system in accordancewith the invention, having a cylindrical conductive or metallictowermember 21 which serves as a waveguide means fortransferring radiofrequency energy from the feed point to the-radiating elements. In theusual arrangement, the'antenna system will be erected vertically and theradio frequency energy will be coupled into the guide near the'bottom.Radiating elements in the form of vertical slots 23 (for producinghorizontally polarized waves) are formed in the wall-of the waveguidenear the top .of the tower 21.

A longitudinal fin or partial septum 25 extends the entire length of theconductive tower 21 and .is positioned on a radius of the cylinder 21.The longitudinal fin 25 is electrically and mechanically connected tothe cylinder 21 and extends substantially one-half the distance acrossthe inside of the tower; that is, its free edge lies approximately alongthe central longitudinal axis of the cylinder. The position of the finorpartial septum 25 may be visualized best by an inspection of Figures 2,3, 4, and 5.

Figure 2 is a cross-sectional view along the line 22 of Figure 1 andshows the configuration of the finned waveguide 21, 25. The electricfield distribution within the waveguide is also indicated in Figure 2.This mode of propagation in a finned cylindrical waveguide is de-' notedthe TEg The maximum concentration. of the electric field on theperiphery of the cylinder 211's opposite the fin 25. No peripheralcurrents exist along this line and a longitudinal slot cut in thecylinder 21 opposite the fin 25 will-not couple to the TEM mode.

Referring now toFigures 3 and 4, there are shown cross-sectional viewsalong the lines 3-3 and 4-'4of Figure 1 of the 'two layers of slots 23spaced apart by an odd multiple of a waveguide half wavelength. Considering Figure 3 first, in the horizontal plane containing the centerof the slot 23'two conductive elements connect ing to the fin 25 areusedto alter the distribution of the electric field inside the cylinderand thus cause the radio frequency energy propagated in-thefinnedwaveguideto couple to the slot; The first of these conductiveelements in a strip of conductive material 27' which extends from thefree edge of the fin 25 to a point on the circumference of the cylinder21. This strip of conductive mate rial ormetal 27 rotates thedistribution of the electric field at the point of connection of thestrip 27 by an amount which is determined byjthe angle/included betweenthe strip ZfT-and the longitudinal fin 25.

The second of these conductive elements utilized to alter I the fielddistribution is a plate 29 which extends toward the wall of the tower 21opposite to that to which the strip 3 27 is connected. This plate 29 mayconveniently be arranged at right angles to the longitudinal fin 25,although of course other angular positions may be utilized as deter--mined by the degree of coupling desired and the impedance matchingconsiderations'of the entire antenna assembly. The dimensions of thestrip 27 and plate 29 may be made identical for all slots in aparticular antenna array, but their orientation to produce in-phaseradiation from all slots will depend upon the spacing between adjacentslots. If adjacent slots 23 in the antenna array are spaced an evenmultiple of a half Wavelength in the finned waveguide, all of the strips27 and plates 29 will be oriented in the same direction; for example,all will be identical to that shown in Figure 3. If the center to centerspacing is an odd multiple of a half wavelength in the waveguide, theorientation of strips 27 and plates 29 will be reversed at adjacentslots so as to produce equal and in-phase radiation from all the slotsin the array.

There may be some applications where a controlled amplitude and phasedistribution (or either one) among the different slots is required. Suchoperation can readily be obtained by proper dimensioning of theindividual stripplate elements 27, 29 behind each slot, and by thespacing between adjacent slots 23 and strip-plate elements 27, 29.

Referring to Figure 4 as well as Figure 3, Figure 4 shows an example ofthe reverse orientation to that shown in Figure 3 which is used toobtain in-phase radiation from slots spaced an odd multiple of a halfWavelength.

Referring to Figure 5, which is a cross-sectional view along the line of5 of Figure 1, there is shown one 9 method of feeding radio frequencyenergy to the finned waveguide tower 21, 25. A coaxial line having aninner conductor 31 and an outer conductor 33 terminates in a probe 35which may be positioned on the same diameter occupied by the partialseptum or fin 25. The inner conductor 31 of the coaxial line isconductively connected to the probe 35, and the outer conductor 33 isconductively connected to the conductive cylinder 21. The shape andlength of the probe 35 and its orientation around the periphery of thecylinder 21 are used to aid in matching the impedance of the coaxialline 31, 33 to the finned waveguide 21, 25.

In Figures 6A and 613 there is shown a top sectional view and a view inperspective respectively on one arrangement for exciting the TE mode inthe finned waveguide 21, 25 using a rectangular waveguide 37. The TEmode is propagated in the rectangular waveguide 37 with the broad wallsof the rectangular waveguide 37 oriented parallel to the longitudinalaxis of the cylindrical waveguide 21. The energy is introduced through arectangular opening in the finned waveguide 21, 25 near but to one sideof the partial septum or fin 25.

Figure 7 is an alternative way of exciting the "PE mode in the finnedwaveguide 21, 25 by means of a rectangular waveguide 37'. In thearrangement of Figure 7, the rectangular waveguide 37 is joined to anopening in the finned cylindrical waveguide 21, 25, the center of theopening shown, but not necessarily being diametrally opposed to thepoint of connection aof the fin 25. In this arrangement, the broad wallsof the rectangular waveguide 37 are in planes transverse to thelongitudinal axis of the cylinder 21 and the narrow walls are parallelto that axis. Energy is introduced into the rectangular waveguide 37 inthe TE mode, the same as in Figure 6, but the rectangular guide 37' isrotated through an angle of 90 In all three feed methods shown inFigures 5, 6, and 7,

diflierent orientations of the feeder transmission line around theperiphery of the cylinder 21 may be utilized to aid in obtainingbroadband matched transitions. Additionally another set of strip-plateelements may be included between the lowest slot 23 and the feedpoint ofthe guide 21 for the purpose of broadbanding the input admittance.

'4 A suitable diameter of a waveguide cylinder like that shown inFigures 1 through 7 is given by the equation where is the frequency inmegacycles at the center of the desired band and D is the diameter infeet. This choice of size gives a ratio of operating frequency tocut-01f frequency of approximately 1.30 and a ratio of wavelength in airto the wavelength in the guide of about 0.63. These parameters allow aslot spacing of approximately 0.8 free space wavelengths for electricalhalf wave spacing of the slots in the guide. With the diameter ofcylinder described, only the dominant mode, that is, the TE canpropagate. The waveguide is below cut-off frequency for all other modeswhich might be spuriously generated.

The length of each slot 23 of Figures 1, 3 and 4 is chosen to be selfresonant at the center of the operating frequency band. The slotsthemselves are preferably wide; good characteristics have been observedfor slots having a ratio of length to width of approximately six to oneand with an overall length of about 0.44 of a free space wavelength.

When coupled to the finned waveguide 21, 25 by means of a strip 27 andplate 29, the slot 23 accepts and radiates some of the energy passing byin the waveguide. The effect of each slot is to shunt an admittanceacross the waveguide at the center of the slot. This admittance exhibitsproperties typical of a series resonant circuit, and the magnitude ofthe admittance is readily controlled by altering the mechanicalconfiguration of the strip 27 and plate 29. The slot admittance isreduced to zero (as long as the slot is diametraily positioned withrespect to the point of connection of the fin 25) if there is no stripor plate to alter the field in the vicinity of the slot. The slot insuch case does not aifect propagation in the waveguide, and no radiationtakes place.

Besides controlling the coupling between the slot 23 and the waveguide21, 25, the strip 27 and plate 29 each present a susceptance load to thewaveguide energy. The strip 27 behaves as a shunt inductance, and theplate 29 as a shunt capacitance. The two together are adjusted so as tobe approximately anti-resonant at the design frequency in the absence ofa slot. Thus when a self resonant slot is placed at the level of thestrip-plate elements, a pure conductance is shunted across thewaveguide. With the arrangement described, a very broad band terminationis obtained, since the susceptance slope of the admittance due to theslot is opposite to the susceptance slope of the strip-platecombination, and therefore a partial cancellation takes place.

The closed portion of the Waveguide 21, 25 above the top slot 23 andbelow the feed point (the probe in Figure 5 or the rectangular waveguideof Figure 6) is made an odd multiple of a quarter wavelength, measuredfrom the center of the slot or the center of the feed aperture, so as topresent an open circuit admittance. In the case of the waveguidecoupling arrangement of Figure 7, the closed end of the waveguide isspaced an even number of quarter wavelengths from the center of the feedaperture.

Figure 8 shows another cross-sectional configuration of waveguide withwhich the principles of the invention may be practiced. The circularwaveguide 21 is divided by a septum 39 extending entirely across adiameter of the cylindrical guide 21. Two partial septa 25' extend fromthe center of the full septum 39 to form a structure consisting of twofinned semi-circular Waveguides with their flat sides adjacent. Slots 23are cut in the conductive cylinder 21 opposite to the attachment of thepartial septa 25' to the common Wall 39 of the two waveguides. Theseslots 23 are excited by conductive strips 27 and plates 29 in the mannerexplained above with reference to Figure 3 for the embodiment with acircular cross-section.

Figure 9 illustrates how the principles of the invention Figure 10 showsanother modification of slotted waveguide which utilizes the slotfeeding principles of the invention. A circular tubular guide 21 isdivided by a total septum 39 into two waveguides of semi-circularcross-section. The slots cut in the periphery of the tubular guide 21are located at points of maximum electric field potential and where theperipheral currents are normally zero. Strips 27 extend from near thecenter of the total septum 39 to a point on the periphery of thecircular tube 21. Plate elements 29' extend from the septum 39 toward,but still spaced from, the wall'21 on the opposite side of the slot 23from which the conductive strips 27 are positioned. Strip and plateelements of Figure 10 act-to alter the field distribution inside thesemi-circular guides in a manner like that explained above in connectionwith Figures 1 and 3 so that the slots 23 in the outer wall of thewaveguide are coupled to the energy propagated therethrough.

Figure 11 shows a triangular waveguide 21' having a slot 23 cut in onecorner thereof. A conductive strip 27 and a plate element 29 arepositioned on the wall of the waveguide opposite the slot 23 to alterthe'field distribution and operate in the same manner as the similarlynumbered elements of Figure 10.

As an example of an antenna in accordance with the invention constructedfor very-high-frequency television service, the following dimensions arerepresentative: the cylinder 21 for operation on television channel 4(66 to 72 megacycles) has anoutside diameter of 6 9 /2 and isconstructed of A" thick mild steel plate, giving an inside diameter of6' 9". The total length from the base of the antenna to the top of thetower is 75. The slots 23 are 6' 4" long by l 1" wide and are spaced llapart, center to center. The center of the top slot is spaced 6" fromthe'closed upper end of the cylinder 21. The septum or fin 25 is 7 mildsteel 40 /2 wide and continuously butt-welded to the inside of thecylindrical tower 21. The strip and plate are also of 7 mild steel andarelO" high, positioned at the center of each slot 23. The strip 27makes an angle of 71 with the septum 25 while the plate 29 makes anangle of 90, extending a distance 1 3 toward the opposite side of thecylinder. The coaxial line 31, 33 is 1 /6 outside diameter 51 /2 ohmline and the probe 35 has an overall length of'32 from the inner wall ofthe cylinder 21 extending toward the septum 25.

The antenna thus constructed in accordance with the invention hascertain important practical advantages. Since it is its own structuralsupport it does not require a separate tower. If made very high, it maybe guyed at any point below the lowest layer of slot radiators. Further,it is unlikely that electrical troubles could develop in the antennaportion after installation, since there are no friction contacts, noinsulating materials, no dissimilar metals to create differentialexpansion and corrosion difficulties, and no de-icing equipment isnecessary. The transmission line eificiency is higher than that which isobtainable by even the largest commercial coaxial or two-wiretransmission line. The transmission line itself, as well as the slotradiators, have a power handling ability many times that which it iscontemplated will be authorized by government agencies havingjurisdiction over frequency and power allocations for broadcast serv-1ces.

What is claimed is:

1. A tubular waveguide adapted to interchange energy between theinterior thereof and surrounding space, said waveguide comprising atubular conductive wall and a longitudinal conductive septum attached tothe interior of said tubular wall, said tubular wall being provided witha longitudinal slot located in the portion of said wall farthest removedfrom the attachment of said septum to said wall, a conductive stripconnected between a point on said wall and a point on said septum, saidconductive strip being located near the center of said slot in thelengthwise dimension of said waveguide.

2. A tubular waveguide adapted to interchange energy between theinterior thereof and surrounding space, said waveguide comprising atubular conductive Wall and a I longitudinal conductive septum attachedto the interior of said tubular wall, said tubular wall being providedwith a longitudinal slot located in the portion of said wall farthestremoved from the attachment of said septum to said wall, a conductivestrip connected between a point on said wall and a point on said septum,a further conductive plate member attached to said septum and extendingtoward but having a free end spaced from said tubular wall in a portionon the other side of said slot from that to which said strip isconnected, said conductive strip and said further conductive platemember being located near the center of said slot in the lengthwisedimension of said waveguide.

3. A tubular waveguide antenna comprising a tubular metallic wall and alongitudinal metallic septum extending the entire length of said tubularwall, said septum being attached at one of its edges to said tubularwall and having its other edge located near the center axis of saidtubular wall waveguide, said tubular wall having a plurality oflongitudinal slots diametrally disposed with respect to the point ofattachment of said septum, a metal lic strip connected between a pointon said metallic wall spaced from the point of attachment of said septumand said other edge of said septum near the center of the length of eachof said plurality of slots, and a further conductive plate memberextending from the connection between said septum and said strip towardbut having a free end spaced from a portion of said tubular wall on theother side of said slot from that to which said strip is connected.

4. A tubular waveguide adapted to interchange energy between theinterior thereof and surrounding space, said waveguide comprising atubular metallic wall and a longitudinal metallic septum attached at oneof its edges to said tubular wall and having its other edge located nearthe central axis of said waveguide, said tubular wall having alongitudinal slot therein diametrally disposed with respect to the pointof attachment of said septum, a metallic strip connected between a pointon said metallic wall and said other edge of said septum and making anangle with said septum in a plane transverse to said central axis ofsaid guide at the center of said slot.

5. A tubular waveguide adapted to interchange energy between itself andsurrounding space, said waveguide comprising a tubular metallic wall anda longitudinal metallic septum attached at one of its edges to saidtubular wall and having its other edge located near the central axis ofsaid waveguide, said tubular wall having a longitudinal slot thereindiametrally disposed with respect to the point of attachment of saidseptum, a metallic strip connected between a point on said metallic walland said other edge of said septum and making an angle with said septumin a plane transverse to said central axis of said guide at the centerof said slot, and a further conductive plate member in said plane havinga free end spaced from said wall but extending toward a portion of saidwall on the other side of said slot from that to which said strip isconnected.

6. A tubular waveguide antenna comprising a tubular metallic wall and alongitudinal metallic septum extending the entire length of said tubularwall, said septum being attached at one of its edges to said tubularwall and having its other edge located near the center axis of saidtubular Wall waveguide, said tubular wall having a plurality oflongitudinal slots oppositely disposed with respect to the point ofattachment of said septum, a metallic strip connected between a point onsaid metallic wall spaced from the point of attachment of said septumand said other edge of said septum near the center of the length of eachof said plurality of slots, a further conductive plate member attheconnection between said septum and each of said strips extendingtoward but having a free end spaced from said tubular wall on the otherside of said slot from that to which each of said strips is connected,and means remote from said slots for coupling transverse electric wavesbetween said waveguide and a further transmission line.

7. A tubular waveguide adapted to interchange energy between itself andsurrounding space, said waveguide comprising a tubular metallic Wall anda longitudinal metallic septum attached at one of its edges to saidtubular wall and having its other edge located near the central axis ofsaid waveguide, said tubular wall having a plurality of longitudinalslots therein diametrally disposed with respect to the point ofattachment of said septum, said slots being spaced apart along thelength of said waveguide, a metallic strip connected between a point onsaid metallic wall and said other edge of said septum in a planetransverse to said central axis of said guide at the center of each ofsaid slots, a further conductive plate member extending from theconnection of each of said strips and said septum toward but having 'afree end spaced from. a portion of said tubular Wall on the other sideof said slot from that to which each of said strips is connected, and aprobe entering said waveguide remote from said slots for couplingtransverse electric Wave energy, said probe being positioned on theopposite side of said waveguide from and spaced from said septum.

8. A tubular waveguide adapted to interchange energy between itself andsurrounding space, said waveguide comprising a tubular conductive walland a longitudinal conductive septum attached to said tubular wall, saidtubular wall having a longitudinal slot in the portion of said wallfarthest removed from the attachment of said septum to said wall, aconductive strip connected between a point on said wall, and a point onsaid septum, a further conductive plate member attached to said septumand extending toward but having a free end spaced from said tubular wallin a portion of said wall on the other side of said slot from that towhich said strip is connected, said conductive strip and said furtherconductive plate member being located near the center of said slot inthe lengthwise dimension of said waveguide, and

8 means remote from said slot for coupling transverse electric waveenergy having a maximum electric field between said septum and theopposite wall portion into said waveguide.

9. A tubular waveguide adapted to interchange energy between theinterior thereof and surrounding space, said waveguide comprising atubular conductive wall and a longitudinal conductive plate attached tothe interior of said tubular wall, said tubular wall being provided witha longitudinal slot located in said wall, a conductive strip connectedbetween a point on said wall and a point on said plate, said pointsbetween which said conductive strip is connected being positioned in aplane that is substantially transverse to the longitudinal axis of saidwaveguide and that passes through said slot.

10. A tubular waveguideantenna comprising a tubular metallic wall, arectangular metallic partition plate positioned within said tubular wallin a plane in which the central longitudinal axis of said tubular walllies, said plate extending the entirelength of said wall and beingattached at one of its longer edges to said tubular wall, the otherlonger edge of said partition plate being physically spaced from saidtubular wall, said tubular wall having a plurality of longitudinal slotstherein positioned with their centers substantially in said plane atpoints in said tubular wall remote from said one longer edge of saidpartition plate, a metallic strip connected between a point on saidtubular wall and a point on said other edge of said partition plate inthe region of each of said slots, said points between which each of saidmetallic strips is connected being positioned in a plane that issubstantially transverse to said longitudinal axis and that passessubstantially through the center of its respective slot, and a metallicextension member fastened to said partition plate adjacent each of saidmetallic strips, each of said extension members extending away from saidpoints of attachment of said strips to said tubular wall, said extensionplates being physically spaced from said tubular wall.

References Cited in the file of this patent UNITED STATES PATENTS2,477,510 Chu July 26, 1949 2,573,461 Lindenblad Oct. 30, 1951 2,591,695Hansen Apr. 8, 1952 2,635,188 Riblet Apr. 14,1953 2,660,674 Brown Nov.24, 1953

